1. Technical Field
This invention is related to photovoltaic devices and, more specifically, to multi-bandgap, tandem, photovoltaic energy converters.
2. State of the Prior Art
It is well-known in photovoltaics that more efficient conversion of solar energy to electrical energy can be accomplished by matching the photon energy of the incident solar radiation to the amount of energy needed to excite electrons in the semiconductor material to transcend the bandgap from the valence band to the conduction band. Such matching avoids energy loss or waste due to the thermalization of excess photon energy over the amount of photon energy that is absorbed by the semiconductor for photon to electric energy conversion and due to insufficient photon energy to be absorbed and converted to electric energy by the semiconductor material. However, since solar radiation usually comprises a wide range of wavelengths, use of only one semiconductor material with one bandgap to absorb such radiant energy and convert it to electric energy results in large inefficiencies and energy losses to unwanted heat.
Ideally, there would be a semiconductor material with a bandgap to match the photon energy for every wavelength in the solar radiation. That kind of device is impractical, if not impossible, but persons skilled in the art are building monolithic stacks of several different semiconductor materials with different bandgaps into devices commonly called tandem converters and/or monolithic, multi-bandgap, tandem converters to get two, three, four, or more discrete bandgaps spread across the solar spectrum to match more closely to at least several different wavelength bands of radiation and, thereby, achieve more efficient conversion of radiant energy to electric energy. Essentially, in such devices, the radiation is directed first into higher bandgap semiconductor materials, which absorb shorter wavelength, higher energy portions of the incident radiation for conversion to electric energy, while the longer wavelength, lower energy portions of the radiation pass through such higher bandgap materials to lower bandgap materials, where they are absorbed and converted to electric energy. Therefore, the higher energy portions of the incident radiant energy are absorbed and converted to electric energy by the higher bandgap semiconductor materials in the stack without excessive thermalization and loss of energy in the form of heat, while the longer wavelength, lower energy portions of the radiation are transmitted to one or more subsequent semiconductor materials with smaller bandgaps for further selective absorption and conversion of remaining radiation to electrical energy.
Persons skilled in the art have developed various techniques for determining or “modeling” theoretical optimum discrete subcell bandgap combinations of two, three, four, five, or more cascaded subcells in a stack for extracting maximum electrical power from specific solar spectrums, e.g., standard solar spectrums AM0 (space above earth's atmosphere), AM1.5 (average sunny day on earth), AM2 (average sunny day at sea level), AOD85 (terrestrial concentrator), etc., at selected operating conditions, e.g., concentration ratio and operating temperature of the solar cell. See, e.g., M. W. Wanlass and D. S. Albin, “A Rigorous Analysis of Series-Connected, Multi-Bandgap, Tandem Thermophotovoltaic (TPV) Energy Converters,” Proc. Sixth Conference on Thermophotovoltaic Generation of Electricity (TPV6), Freiburg, Germany, Jun. 14-16, 2004, AIP conf. proc. 738, pp. 462-470. and M. F. Lamorte and D. H. Abbott, “Computer Modeling of a Two-junction, Monolithic Cascade Solar Cell,” IEEE Transactions on Electron Devices,” Volume 27, Issue 1, January 1980, pages 231-249. There are also several computer programs available for use in modeling crystalline solar cells, for example, PC1D from the University of South Wales, Sidney, Australia, and Sim Windows from the University of Colorado, Boulder, Colo. USA. However, such modeling only provides the theoretical optimum subcell bandgaps for the solar spectrums, concentration ratios, and operating conditions. The challenge then is to find ways to grow the subcells continuously, and, further, to mitigate cost and make them durable.
Semiconductor compounds and alloys with bandgaps in the various energy ranges needed for efficient solar to electric energy conversion are known or achieveable, but that knowledge alone does not solve the problem of how to make the most efficient and useful solar to electric energy conversion devices. Defects in crystalline semiconductor materials, such as impurities, dislocations, and fractures provide unwanted recombination sites for photo-generated electron-hole pairs, resulting in decreased energy conversion efficiency. Therefore, high-performance, photovoltaic conversion cells comprising semiconductor materials with the desired bandgaps often require high quality, expitaxially grown crystals with few, if any, defects. Growing the various structural layers of semiconductor materials required for a multi-bandgap, tandem, photovoltaic (PV) conversion device in a monolithic form is the most elegant, and possibly the most cost-effective, approach.
Epitaxial crystal growth of the various compound or alloy semiconductor layers with desired bandgaps is usually most successful, when all of the materials are lattice-matched (LM), so that the semiconductor materials with layer crystal lattice constants are not interfaced with other materials that have smaller lattice constants or vice versa. Lattice-mismatching (LMM) in adjacent crystal materials causes lattice strain, which, when high enough, is usually manifested in dislocations, fractures, wafer bowing, and other problems that degrade or destroy electrical and other characteristics and capabilities of the device. Unfortunately, the semiconductor materials that have the desired bandgaps for absorption and conversion of radiant energy in some energy or wavelength bands do not always lattice match other semiconductor materials with other desired bandgaps for absorption and conversion of radiant energy in other energy or wavelength bands.
Co-pending U.S. patent application Ser. No. 10/515,243, which is incorporated herein by reference, addressed this problem primarily for low bandgap (e.g., less than 1.35 eV in this context), monolithic, multi-bandgap devices in order to convert lower energy, infrared radiation (e.g., ˜927 to 3,483 nm) to electricity more efficiently. By the use of a combination of subcells lattice-matched (LM) to InP substrates, lattice constant transition layers, and lattice-mismatched (LMM) subcells, inverted monofacial and bifacial structures, ultra-thin monolithic, multi-bandgap, tandem structures, and other features, the inventions in that co-pending patent application could provide monolithic, multi-bandgap, tandem energy converters with subcell bandgaps in various combinations ranging from about 1.36 eV down to as low as about 0.35 eV. That range comprises invisible infrared energy.
Another co-pending U.S. patent application Ser. No. 11/027,156, which is also incorporated herein by reference, shows an inverted, ultra-thin design for a monolithic, multi-bandgap, tandem, photovoltaic converter combining lattice-matched (LM) medium and high bandgap materials with lattice-mismatched, low bandgap materials. The ability to include a low bandgap (less than about 1.2 eV) subcell in a monolithic, tandem, stack device along with medium (about 1.2 to 1.6 eV) and high (about 1.6 to 2.2 eV) facilitated achievement of higher solar to electric energy conversion efficiencies. However, while such higher conversion efficiencies facilitated by that design are significant, the design still has limitations and problems. For example, the bandgap for the lattice-matched (LM) Group III-V subcell GaInP is limited to about 1.9 eV for the top or front subcell on conventional, industry standard substrates, such as GaAs and Ge, unless Al is added to the GaInP, which typically introduces material problems that are usually best avoided. This practical limitation of 1.9 eV is significant, because it allows a substantial amount of solar energy in the shorter, more energetic, wavelength ranges of the solar spectrum, i.e., less than about 600 nm to be lost to thermalization.
Also, the lattice-mismatched subcells cause epiwafer bowing that can be problematic when growing devices on large substrates and when processing large devices even where graded layers are used to facilitate transition between materials with different lattice constants. This problem can be exacerbated in situations wherein the spectral range of the optimum bandgaps for maximum solar energy to electrical energy is broad, for example in converters with three or more subcells.
Other practical problems can also be encountered where the spectral range of the optimum bandgaps is broad. For example, typical antireflective coatings containing two or three layers cannot achieve low reflectance, i.e., less than a few (2-3) percent from the blue end of the solar spectrum all the way to about 0.7 eV infrared radiation, which is approximately the bandgap of active Ge bottom subcells used in state-of-the-art tandem solar cells. So far, better antireflective coatings able to achieve that kind of low reflectance over that kind of spectral range have not been reported.
Consequently, because of the practical problems associated with the broad spectral ranges of the optimum bandgaps in solar energy converters with three or more subcells, the maximum efficiencies predicted by the models that indicate such bandgaps have not been obtainable.